The present invention relates to the control of the execution of a load for a storage, particularly the control of parallel processing with respect to input and output for disk drives.
In Japanese Patent Laid-Open No. 114947/1985, a double write control has a cache (hereinafter referred to simply as a cache). Two disks, called double write disks, are each written with the same data. A control unit processes an input output request from a CPU for one of the two disk units. In the case of receiving a read request (input request) from the CPU, the control unit executes the request as it is. In the case of receiving a write request (output request) from the CPU, data is written in a specific one of the double write disks and at the same time the same data is written in the cache. At a later time, making use of available processing time when the control unit and disks have nothing else to do, the control unit writes the same data from the cache into the other disk unit, which is called a write after process. In this manner, the same data is written to each disk unit of the double write disk units.
In Japanese Patent Publication No. 28128/1986, there is disclosed a double filing control for load distribution with respect to double write disk units. There is no write after process. The control is designed to achieve a higher processing speed by selecting an inactive disk unit, among the disk units, when an input/output request is received. An inactive disk unit will be defined herein as a disk unit that is currently not undergoing any disk accessing, that is not undergoing any read or write operation.
In a thesis found in the Information Process Institute Bulletin "Nishigaki et al: Analysis on Disk Cache Effects in a Sequential Access Input Process", Vol. 25, No. 2, pages 313-320 (1984), there is disclosed with respect to a single disk unit a read ahead control having a cache, which involves the staging, in the cache, data not requested by the CPU but which will be requested in an instruction shortly following the current instruction. This staging process is executed by the control unit independently of any execution of an input/output request from the CPU.